Explain motion and energy before collisions

9.A Apply scientific ideas and evidence to construct an explanation for the causes of motion and kinetic energy changes that happen before and after collisions and how these affect the outcome of a collision.

As you formatively assess student explanations look for these ideas:

For friction-related explanations

• When surfaces in contact with each other slide along each other, the bumps (roughness) on their surfaces cause an interaction (effect).
• That interaction (cause) results in the surface deforming (effect).
• That deformation (cause) results in that part of the matter pushing back on the object it is in contact with (effect).
• This force applied against the moving object is friction. It causes the cart or box to slow down (effect).
• These contact forces (cause) cause energy to be transferred to the surfaces in contact with each other, resulting in an increase in the particle-level kinetic energy (an effect measurable by a temperature increase).

For air resistance-related explanations

• When an object moves through air (cause), it collides with air particles (effect).
• These collisions cause contact forces (effect) on both the object and the air particles.
• The force applied against the moving object is air resistance. It causes the cart or box to slow down (effect).
• These contact forces (cause) cause energy to be transferred to the surrounding air, moving it out of the way of the cart (or speeding up those particles).
• In the interaction with the cart being pushed by a headwind, the wind is a resistant force and KE is expelled on the wind.
• In the interaction with the cart being pushed by a tailwind, the wind is not a resistant force but adds KE to the overall cart system.
• There often is more than one contact force on a moving object (such as surface friction, air resistance, and wind).